Does anyone know any links where i can learn how to properly set up a .arena file so a level appears in skirmish menu, and also has 2 or 3 bots listed?

It's very simple.
You can also extract the contents of a .pk3 file and check out a .shader file in the scripts folder via any text editor.

The format is something like this (standard Quake 3):

{
map "map_name"
bots "bot_names"
longname "long_map_name"
type "game_mode"
}

There are other variables such as timelimit and fraglimit which you can also add but they aren't mandatory. They follow the same format convention.

Here is an example:

{
map "bloodq3dm1"
bots "major sorlag doom"
longname "My Map (by Blood)"
type "ffa tourney team"
}

Great, thanks for your help PaN61 :)

If you want to list your map in Worldspawn (ws.q3df.org), you will need to include also the author "your_nick" in the .arena file code to specify the author of map. Worldspawn uses the .arena file to identify the map name, longname, gametype and even author. Worldspawn is an automatic site which adds your map immediatelly after you send it in. It is good for DeFrAg map makers, who need to quickly release maps and want to get it to most people, who ever do not visit his sites or he doesn't have sites... It's an analogous to the quick-pace of the DeFrAg maps, which are designed in different way compared to Vanilla Q3. Also DeFrAg maps do not use .arena file to define map's gametype and other properties including author, but the .defi file, which is pretty similar to *.arena file, but has different properties and gametypes are different, designed more likely to look like Olympics sports. These maps do not support bots (this support ability was also removed from that mod at all) and all you need to do is just to run and use weapons to enhance your speed. These maps are called "speedruns". Also shooting is avaliable and is labeled "accuracy" You need to shoot specific amount of targets in shortest time. Your amiming skills are very important, so these maps contain only Railgun and some bunch of Slugs ammo boxes...

If you want just map to Quake 3 and do not want to quickly release maps and instead want to properly get that map reviewed, this site is the right choice ;) This is the reason why I moved here :)